Particulates or particulate matter entrained in exhaust gas from a diesel engine is mainly constituted by carbonic soot and a soluble organic fraction (SOF) of high-boiling hydrocarbons and contains a trace of sulfate or misty sulfuric acid fraction. In order to reduce a discharged amount of particulates from the engine, a particulate filter is conventionally employed and incorporated in an exhaust pipe through which the exhaust gas flows.
The particulate filter of this kind is a porous honeycomb structure made of ceramics such as cordierite and having lattice-like compartmentalized passages; alternate ones of the passages have plugged inlets and the remaining passages with unplugged open inlets are plugged at their outlets. Thus, only the exhaust gas passing through the thin porous compartment walls is discharged downstream.
The particulates entrained in the exhaust gas, which are captured and accumulated on the inner thin porous compartment walls of the particulate filter, require to be burned off so as to regenerate the particulate filter before exhaust resistance considerably increases due to clogging. However, the exhaust gas from the engine in a normal operation status rarely has a chance to reach a temperature level at which the particulates ignite by themselves. Thus, a catalytic regenerative particulate filter has been developed into practical use which integrally carries an oxidation catalyst made from alumina carrying platinum and added with a required amount of rare-earth element such as cerium.
Use of such catalytic regenerative particulate filter facilitates oxidation of the captured and accumulated particulates and lowers their ignition temperature so that the particulates can be burned off even at an exhaust gas temperature lower than before.
However, even in use of such catalytic regenerative particulate filter, a captured amount of particulates may exceed a burned-off amount of particulates in engine operation areas with low exhaust temperature level; and continued engine operation with such low exhaust temperature level may hinder satisfactory proceeding of regeneration of the particulate filter, resulting in excessive accumulation of the captured particulates in the particulate filter. So, it has been envisaged that fuel is added into the exhaust gas upstream of the particulate filter to forcibly regenerate the particulate filter before the particulates are substantially accumulated.
Specifically, the fuel added to the exhaust gas upstream of the particulate filter is oxidized on the oxidation catalyst carried by the filter to produce reaction heat and raise the temperature of the catalytic floor to burn off the particulates, thus regenerating the particulate filter.
This kind of forcible regeneration of a particulate filter is disclosed, for example, in the undermentioned References 1 and 2 with the applicant same as that of the present invention.    [Reference 1] JP 2003-83139A    [Reference 2] JP 2003-155913A
Particularly in an exhaust gas emission control device with a flow-through type oxidation catalyst in front of a particulate filter to assist oxidation of captured particulates in the filter, added fuel is oxidized on the oxidation catalyst in front of the particulate filter to produce reaction heat and raise temperature of exhaust gas which is then introduced into the particulate filter; as a result, the particulate filter can be forcibly regenerated at a further low exhaust temperature level.